The great grey owl or great gray owl (Strix nebulosa) is a very large owl, documented as the world's largest species of owl by length. It is distributed across the Northern Hemisphere. In some areas it is also called Phantom of the North, cinereous owl, spectral owl, Lapland owl, spruce owl, bearded owl, and sooty owl.[2]

Adults have a large, rounded head with a grey face and yellow eyes with darker circles around them. The underparts are light with dark streaks; the upper parts are grey with pale bars. This owl does not have ear tufts and has the largest facial disc of any raptor.

In terms of length, the great grey owl is believed to exceed the Eurasian eagle-owl and the Blakiston's fish owl as the world's largest owl.[3] The great grey is outweighed by those two species as well as several others, including most of the Bubo genus.[4] Much of its size is deceptive, since this species' fluffy feathers, large head and the longest tail of any extant owl obscure a body lighter than that of most other large owls. The length ranges from 61 to 84 cm (24 to 33 in), averaging 72 cm (28 in) for females and 67 cm (26 in) for males. The wingspan can exceed 152 cm (60 in), but averages 142 cm (56 in) for females and 140 cm (55 in) for males. The adult weight ranges from 580 to 1,900 g (1.28 to 4.19 lb), averaging 1,290 g (2.84 lb) for females and 1,000 g (2.2 lb) for males.[5] The males are usually smaller than females, as with most owl species.[2]

They breed in North America from as far east as Quebec[2] to the Pacific coast and Alaska, and from Finland and Estonia across northern Asia. They are permanent residents, but may move south and southeast when food is scarce. A small population, estimated at less than 100 birds, occurs in the Sierra Nevada mountains of California. This population is the southernmost population of the species' range and is listed Endangered under California's Endangered Species Act. In Europe they are found breeding in Norway and Sweden and more numerously through Finland and Russia.

Adult female

Their breeding habitat is the dense coniferous forests of the taiga, near open areas, such as meadows or bogs. Great grey owls do not build nests, so typically use nests previously used by a large bird, such as a raptor. They will also nest in broken-topped trees and cavities in large trees. Nesting may occur from March to May. Four eggs are the usual clutch size. Eggs average 42.7 mm (1.68 in) wide and 53.5 mm (2.11 in) long. The incubation period is about 30 days, ranging from 28 to 36 days. Brooding lasts 2 to 3 weeks, after which the female starts roosting on a tree near nests. The young jump or fall from the nest at 3 to 4 weeks, and start to fly 1 to 2 weeks after this. Most offspring remain near their natal sites for many months after fledging.

The abundance of food in the area usually affects the number of eggs a female lays, a feature quite common in northern owl species. If food is scarce, they may travel a fair distance to find more prey, with considerable movements by large numbers in some years of particularly scarce prey. Though they do not migrate, many are at least somewhat nomadic.

These birds wait, listen, and watch for prey, then swoop down; they also may fly low through open areas in search of prey. Their large facial disks, also known as "ruffs", focus sound, and the asymmetrical placement of their ears assists them in locating prey, because of the lack of light during the late and early hours in which they hunt. On the nesting grounds, they mainly hunt at night and near dawn and dusk; at other times, they are active mostly during the night.

They have excellent hearing, and may locate (and then capture) prey moving beneath 60 cm (2.0 ft) of snow in a series of tunnels solely with that sense. They then can crash to a snow depth roughly equal to their own body size to grab their prey. Only this species and, more infrequently, other fairly large owls from the Strix genus are known to "snow-plunge" for prey, a habit that is thought to require superb hearing not possessed by all types of owls.[6]

The harvest of timber from the great grey owl's habitat is, perhaps, the greatest threat to this species.[7] Intensified timber management typically reduces live and dead large-diameter trees used for nesting, leaning trees used by juveniles for roosting before they can fly, and dense canopy closures in stands used by juveniles for cover and protection.[7] If perches are not left in clearcuts, great grey owls cannot readily hunt in them. Although human-made structures (made specifically for use by this species) have been utilized by these owls, the species is far more common in areas protected from logging. Livestock grazing in meadows also adversely affects great grey owls, by reducing habitat for preferred prey species.[7]

Due to their large size, great grey owls have few natural predators. Great horned owls, various small carnivores, and black bears have been documented preying on young, but such predators rarely threaten adults, and owls have been known to fend off animals as large as black bears when defending their nests.[8] The only known predator of adult great grey owls is the Eurasian eagle-owl (Bubo bubo), which occasionally preys on the former in parts of Europe.[9]